Question

A sample of ammonia (approximately 0.10 M)(pKb=4.74) is titrated with 0.12 M of Hydrochloric Acid. What indicator changes color closest to the equivalence point?

a. thymol blue (pKIn = 1.65)

b. methyl orange (pKIn = 3.46)

c. methyl red (pKIn = 5.00)

d. bromthymol blue (pKIn = 7.30)

e. phenolphthalein (pKIn = 8.7)

Answer 1

First we calculate the pOH of the weak base using Handerson-Hassslbach equation. Then we calculate the pH of weak base from the pOH value. It comes out to be 9.18. Then we plot a general pH vs Volume of Strong Acid added (mL) curve and observe that the curve pH of weak base remains constant at 9.18 for some volume of strong acid, and then the curve start falling again, but less steeply than for a titration of a strong base with strong acid. We got our equivalence point in between the range of 5-6 pH. And the curve flattens again at the pH of strong acid, which in general is around 2.

The pH range of 5-6 for equivalence point best matches with that of Methyl Red (pKIn = 5.0). Therefore, option C. Methyl Red is correct.

The detailed solution is given below: